It has long been known to convert dc power to ac power; and such power converters -- known as inverters -- which convert constant voltage, regulated dc to ac, using SCR's, have been known. However, SCR operated inverters having very high power ratings at low, industrial frequencies (generally 60 Hz or 400 Hz) have tended to be unstable or to have wide frequency variations beyond an acceptable level. Inverters of the prior art have included circuitry having closed loop feedback control where the ac output is sensed and the dc firing is controlled thereby, and have included generations of inverters having phase angle controlled firing, pulse modulated firing, and pulse modulated and phase angle controlled with pulse shaping, etc. For example, the firing angle of transistors or triacs used in a dc-chopper circuit may be controlled so as to assure a constant average output voltage, such as by controlling the pulse width and/or the pulse amplitude, while sensing the ac output and controlling the chopper having the dc input. It is usual, in the prior art inverter technology, to provide combination LC series/parallel resonant ac waveform filters or, in the case of pulse modulated firing, capacitor filters.
However, the prior art inverter circuitry such as that discussed above, has had a number of disadvantages. For example, in using capacitor filters with inverters having other than the very smallest power ratings, filter banks which are physically very large have been required. More especially, however, is the fact that while the electronic sensing of the ac output of an inverter is very fast, and control of SCR firing can be equally as fast -- in the order of a few milliseconds -- the response time of a series/parallel LC filter may be quite sluggish in comparison -- in the order of three to thirty cycles of inverter operation, depending upon frequency and power rating. If the inverter is subject to a dynamic load, i.e. one which may vary rapidly as to its power requirements or its power factor, or if the inverter circuit is subject to fluctuations or sudden changes in its dc input voltage level, the ac output voltage may vary considerably in level and, indeed, in frequency. What may happen, therefore, is that the ac output may tend to overshoot or undershoot, the ac output may ring or the chopper output may ring, hunting may occur and feedback loop instability results. The output may then collapse, the SCR's may misfire, and the inverter fails.
In addition, as the load on an inverter changes or its power factor changes, which may be following by a period of ringing of the inverter output or instability of the feedback and control loops within the inverter, there may be very high power radio frequency noise generated in the dc-chopper. Additionally, the firing circuit for the SCR's in the chopper may, itself, become subject to RF interference.
In the instance when two or more inverters may be connected in parallel, reliability and stability problems are encountered because, for example, it is difficult to predict the precise manner in which an inverter output may ring, or to overcome or suppress loop instabilities. In addition, difficulties occur when two or more inverters are either connected to the same dc source, or even when a single inverter is switched from one dc source to another having a slightly different level. To overcome some of these difficulties, designers have been obliged to assure that each inverter has its own dc supply and that no other noise creating dc loads are connected to that supply.
In addition, none of the prior art inverters provide inherent current limiting and short circuit proof performance; and current limiting in such inverters must be established electronically. This means, of course, that in the case of an electronic failure, SCR's might be short circuited, and diodes and fuses can be overloaded, and damage of failure to the inverter occurs; which, in any event, requires a period of non-operating time for repair or replacement.
Akamatsu, in U.S. Pat. No. 3,683,267 issued Aug. 8, 1972, teaches a power control system having an inverter which includes at least two thyristors or SCR's, and where a non-linear reactor is connected in series with each thyristor. A commutating capacitor is connected between the thyristors, and at any instant one of the thyristors is fired so as to apply the voltage which is accumulated on the commutator capacitor to the other of the thyristors, in the reverse bias direction, so as to effect commutation. However, the Akamatsu circuits are particularly adapted for use at relatively high operating frequencies, and are devised so as to reduce commutation current and absorption of feedback power, thereby increasing the efficiency of the inverter. The Akamatsu circuits, however, do not lend themselves to parallel operation, either from different sources or from the same source; and the circuits are such that the output may, under rapidly changing load conditions, have a high harmonic content.
Wurst, et al, in U.S. Pat. No. 3,702,432 issued Nov. 7, 1972, teach an inverter which uses SCR's, and where bifilar choke is used having one winding in series with each of the SCR's and off the end of a centre-tapped output transformer so as to prolong the discharge time of the commutation capacitor and thereby prevent high peak currents during commutation. The Wurst et al circuits, however, not only require the use of a special choke -- rather than "off-the-shelf" components -- but the Wurst et al circuit may tend to have frequency instability.
What applicant does herein, is to provide a group of inverter circuits -- which may operate either with a centre-tapped transformer connected to a dc-chopper or with a bridge-type chopper across a transformer -- and which in any event have high reliability and a completely isolated dc input circuit from the ac output circuit, including an isolated dc feedback loop. The circuits of the present invention have choppers which operate over 180 electrical degrees on each side so that the inverter output frequency is substantially constant and is independent either of the load or of the input dc voltage.
The circuits of the present invention are particularly adapted to be operated with a ferroresonant voltage regulating circuit of the sort which is taught in applicant's U.S. Pat. No. 3,824,449. In the circuits of the present invention, the commutation capacitor may be decoupled from the dc-chopper transformer, by means of series diodes; and in such circuits, because the 180 electrical degree commutation effectively frees the inverter from effects of changes of power factor of the load, current and voltage stresses within the commutation circuits of the inverter are substantially constant and are predictable.
Other features of the present invention are the inclusion of input circuitry which limits in-rush current and permits very fast start up of the inverter. Such "soft start" circuitry creates less stress on the dc power source, and on the components in the commutation circuits.
Thus, the present invention comprises an inverter circuit, as described above, having the combination of at least:
a. an input diode connected to the dc source for the inverter, and having its polarity arranged so as to be normally conductive; PA1 b. an input filter choke in series with the dc input and the input diode; PA1 c. an input, polarized capacitor across the dc input; PA1 d. a pair of SCR's facing in the same direction with respect to the input and having an unpolarized commutation capacitor facing the SCR's on the side of each thereof which is remote from the dc source; PA1 e. a drive circuit for the SCR's arranged so that, at any time, one or the other of the SCR's is conductive; PA1 f. a transformer -- which is essentially a dc-chopper transformer -- arranged with its primary winding connected so that at any instant of time at least a portion of the primary winding is in series with the SCR which is conductive at that instant -- the ac output of the basic inverter circuitry being taken from the secondary winding of the transformer; PA1 g. a pair of de-coupling diodes, each connected to the primary winding of the transformer so as to have 100% of that winding between their connection points, with the other sides of each of the de-coupling diodes being connected in series to one each of the SCR's; the unpolarized commutation capacitors being connected between the common points of the series connection of the pairs of SCR's and de-coupling diodes; PA1 h. a commutation choke in series with the SCR's and connected in such a manner that each of the SCR's is in series with at least a portion of the commutation choke; PA1 i. and a feedback circuit which comprises at least one diode arranged in counter polarity to the polarized input capacitor and connected to the opposite polarity side of the polarized input capacitor, with the other side of the diode facing the primary winding of the transformer.
The above is a basic description of the principle arrangement of basic circuits according to the present invention. For example, a dc-chopper transformer may be a centre-tapped transformer where each of the SCR's faces one or the other of the ends of the primary winding of the transformer, and the primary winding is centre-tapped to one side of the dc input; or the dc-chopper may be of the bridge variety having two pairs of SCR's where both SCR's of each pair face in the same direction and the pairs are oppositely faced with respect to the dc input, and where each of the SCR's in each pair faces one or the other of the ends of the primary winding of the transformer. In the latter circumstances -- i.e., in the case of a bridge-type dc-chopper -- one or the other of each of the pairs of SCR's is conductive at any instant of time, in a manner so that both ends of the primary winding of the dc-chopper transformer face a conductive SCR at all times.
A number of variations with respect to the basic circuits according to this invention are discussed hereafter. Such variations include direct-coupled commutation capacitors, soft-start input circuits, forced commutation of both centre-tapped chopper circuits and bridge-type chopper circuits, alternative feedback arrangements, etc.